This invention relates to communications systems and in particular to identification apparatus for reading electronic identification tags.
Identification apparatuses have numerous applications where it is advantageous or necessary to identify and/or locate movable objects. For example, such apparatus can be used in in-store security systems to raise an alarm if a product (fitted with a tag) is being removed from a designated area in the store. Alternatively, the apparatus can be used to identify live stock (carrying the tags) in a milking or feeding parlour.
Many of these types of identification apparatus are used with tags which transmit RF identification signals which cooperate with an RF unit in the apparatus.
Several different designs of electronic tags and RF units are available. One type of tag has an in-built battery to provide the power necessary to operate it. This, however, has been found to have limited application.
Alternatively, the tag may have a passive transponder which obtains power from the apparatus by means of an inductive pickup (which may be provided by the antenna which also transmits an RF identification signal), and which therefore does not require a built-in battery.
In a type of system known as full duplex, the tag receives its charging signal and transmits its identitification signal at the same time.
Alternatively, the tag receives its charging signal during one half of an operating cycle and then transmits its identitification signal during the other half. This is known as a half duplex arrangement.
In both of these systems, the tag receives its energy by means of a charging signal which is generated by the Radio Frequency circuit (RF Unit) and transmitted by an antenna on the identification apparatus. Other parts of the RF Unit act as a receiver and decode the information which is transmitted from the tag when charged and received either by the same antenna or another, specialised receiving antenna also connected to the RF Unit.
In one known identification system the RF Unit is designed to be connected to only one antenna which is located within a distance of about one meter to achieve the maximum read range for the tags. The system operates at a frequency of 134.2 kHz and the antenna and its cable must have a total inductance which is between 26 and 28 micro Henrys (nominally 27). The maximum read range with the standard tags is in the order of 1100 mm when operated with an antenna comprising a coil of either Litz wire (consisting of a large number of very fine, insulated strands) or Speaker Cable (Oxygen Free Copper cable similar to Litz wire but without the individual strand insulation).
Typically, the antenna may have a diameter in the range of a few millimeters (if it is included in a hand held unit) to around one meter (if the antenna is to form. part of a static installation).
Larger heavier tags operating in a different manner can achieve read ranges of more than 3 meters but these are not practical for the majority of uses.
However, if the antenna is to be located further away from the RF Unit, it has to have a reduced inductance to compensate for the increased length of cable needed to connect the antenna to the RF Unit, since the increased length of cable will make a larger contribution to the inductance of the load (i.e. the cable and antenna) connected to the RF Unit.
This reduces the range of the antenna and places a number of constraints on the designs of antenna which can be used in the identification apparatus.
According to a first aspect of the invention, there is provided a communications system comprising a driver connected to a load, which includes an antenna, and operable to supply current to the load thereby to cause the antenna to emit power to energise a remote transponder, wherein the load includes an inductor connected in parallel to the antenna and driver so as to reduce the total inductance of the load.
If the only inductances in the load are provided by the antenna and the inductor, then the total inductance, LT, of the load is given by the equation
LT=1/L1+1/L2xe2x88x921
where L1 and L2 are the respective inductances of the antenna and the inductor. Thus if, for example, the driver has to see a load of inductance of 27 micro Henrys, and the inductor has an inductance of around 40 micro Henrys, then the antenna can have an inductance of 83 micro Henrys. Thus the invention enables the driver to be used with an antenna of a larger inductance than would be the case in the absence of the inductor. It has also been found that various design constraints on antennae of impedance less than around 27 micro Henrys do not apply to an antenna having an inductance in the range of 75 to 100 micro Henrys which has been found to be a convenient range of possible inductances for at least one application (described below).
Preferably, therefore, the inductance of the inductor is such that the inductance of the antenna can lie in the range of 74 to 93 micro Henrys and the antenna in this case conveniently takes the form of a rectangular coil 850 mm high and 325 mm wide.
The driver may in addition incorporate a tuning inductor of variable inductance to enable the driver to be used with antennae of differing inductances in a given range. In this case, the parallel inductor in the load increases the range of possible inductances of the antennae which can be used, as well as allowing antennae of large inductances to be used.
Where the system is adapted to operate with a load of approximately 27 micro Henrys, the inductor preferably has a inductance of approximately 50 micro Henrys or less, preferably 40 micro Henrys, to enable the system to be used with an antenna having an inductance within the range of 74 to 93 micro Henrys.
Preferably, the inductor is positioned adjacent the driver, so that any cable connecting the antenna to the driver is also connected in parallel with the inductor to the driver.
As a result of this feature, the contribution of the inductance of the length of cable to the total inductance of the load can be reduced. Consequently, the invention also enables the antenna, to be connected to the driver by a relatively long length of cable, and thus to be situated a relatively large distance away from the driver, whilst the inductance of the cable places only minor (or no) practical constraints on the size of the antenna.
Theoretically, it should be possible to use an inductor with an inductance only slightly greater than the required total inductance of the load (for example 27 micro Henrys), and connected in parallel to an antenna having an inductance of several hundred or even thousands of micro Henrys. In practice, the performance of a coiled antenna improves with the number of turns in the coil only until the voltage which is developed across the antenna (and hence the range of the antenna) is limited by the resistance of the wire from which the antenna is constructed. However, the invention does allow the system to be used with a wide range of different types and sizes of antenna constructed from good quality coiled power cable. Generally, the larger the antenna, the fewer the number of turns needed to achieve a given range.
However, again, the resistance of the wire forming the antenna does impose a limit on the maximum voltage and read range which can be achieved.
Preferably, the antenna is connected to the driver by a length of coaxial cable.
Coaxial cable is particularly advantageous since it is relatively cheap, generates low levels of noise and only gives rise to a small degree of antennuation of voltages supplied along the cable.
The system may to advantage be so arranged that the signal emitted by the antenna energises a remote transponder, causing the latter to emit a identification signal. Conveniently, that signal is received by the same antenna and read by the driver. The system therefore functions as identification apparatus for reading and identifying each of a plurality of identification transponders.
The system may have a single antenna connected to the driver and inductor (which is optionally adjacent the driver).
Alternatively, the antenna may be one of a plurality of such antennae, each of which is connected to the driver and the inductor (which is optionally adjacent the driver) by a respective piece of cable.
Preferably, the driver is connected to each cable by a respective switch, and the switches operate in sequence so that the driver is connected to each antenna in turn.
If the antennae are placed at different distances from the driver, the inductances of their connecting cables will correspondingly differ. However, since the cable, and its antenna, is connected in parallel to a further inductor, the effect of those variations in inductance on the overall inductance of the load seen at the driver will be reduced.
Preferably, all of the antennae and corresponding cables are connected in parallel to the same inductor.
This feature is of particular advantage if the system is to be used in an environment, such as in a milking or feeding stall, which is potentially hostile to electronic equipment, since the driver and the switches can be grouped together in a protective housing, whilst the antennae are situated in relatively exposed locations in the stalls.
The invention also lies in identification apparatus for reading and identifying each of a plurality of transponders arranged to emit corresponding identification signals, the apparatus comprising a plurality of antennae for receiving said signals and/or transmitting sufficient power to energise said transponders, and a driver connected to the antennae and operable to supply said power thereto, wherein each antenna is connected to the driver by a respective cable.
Preferably, the driver is connected to each cable by a respective one of a number of switches, such as relays, located adjacent the driver.
The invention also lies in a transponder for a communications system, the transponder comprising an antenna for receiving power, circuitry connected to a load which includes the antenna, which is operable to store power picked up by the antenna for use in the subsequent operation of the transponder, wherein said load includes an inductor connected in parallel with the antenna and the circuitry so as to reduce the total inductance of the load as seen by the circuitry.
The antenna in any of the aforementioned systems may comprise a coiled antenna, and may comprise a plurality of coil elements, provided that the total inductance and resistance of the load complies with the requirements of the unit or circuitry to which it is connected.